Method for producing HBr

ABSTRACT

Method and apparatus are disclosed for producing oxygen and hydrogen bromide in an electrolytic cell utilizing, as the cathode, a consumable bromine electrode comprising bromide ions dissolved in a pool of liquid bromine surrounding a metal electrode. The electrolytic cell also contains a water solution of an electrolyte on both sides of a hydrogen ion permeable membrane forming a two-compartment electrolytic cell. In operation of the preferred process, hydrogen bromide gas is given off in the cathode compartment and oxygen gas is given off in the anode compartment.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to an application filed by the same inventor, havingthe same assignee, Ser. No. 956,760 of even filing date herewithentitled "Hydrogen Gas Generation Utilizing A Bromide Electrolyte AndRadiant Energy" which demonstrates a method of evolving hydrogen gasutilizing hydrogen bromide which can be produced by practice of theinstant invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of art to which this invention pertains is electrolyticprocesses and, particularly, those for producing hydrogen bromide andelectrolytic cells useful for peforming such processes.

2. Description of the Prior Art

The recognition of the potential for using liquid bromine as anelectrode in the prior art has received little attention because of theelectrical insulating properties of liquid bromine, i.e., it has notbeen useful for transmitting electric current in the electrolytic cellenvironment. And while hydrogen bromide has been recognized as havinggreat potential in a fuel cell environment, it has received littleattention because of the difficulties of producing it, especially in anelectrolytic cell. Note an article entitled "Performance OfHydrogen-Bromine Fuel Cells" by Werner Glass et al, Advances inChemistry Series, Vol. 47, 1964, ACS Applied Publications. One approachto the use of hydrogen bromide in an electrolytic and ultimately fuelcell environment is disclosed by Meyerand et al, U.S. Pat. No.4,069,120. In this patent, bromine and water are reacted in gaseous formto produce hydrogen bromide and oxygen. However, as noted, suchreactions take place in the gaseous phase.

Accordingly, what has been lacking in the prior art is a simpleelectrolytic method of producing hydrogen bromide in a simple, basicapparatus.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a metal electrode surroundedby a pool of liquid bromine containing dissolved bromide ions isutilized in an electrolytic method to produce hydrogen bromide.

Particular embodiments of the invention are demonstrated by severalversions of apparatus useful to perform such processes.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof as discussed andillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of apparatus useful for performing the inventionutilizing a battery as a power source.

FIG. 2 is a drawing of apparatus similar to that shown by FIG. 1utilizing radiant energy as the power source.

DESCRIPTION OF PREFERRED EMBODIMENTS

As discussed above, the invention provides an efficient, simple methodand apparatus for producing hydrogen bromide and oxygen in anelectrolytic cell. The hydrogen bromide and oxygen evolved from suchcell can be used ultimately to produce hydrogen and oxygen for use in afuel cell.

The basic reactions involved in the method and apparatus of theinvention may be expressed as follows:

at the cathode,

    Br.sub.2 +2e.sup.- →2Br.sup.-

    Br.sup.- +H.sup.+ →HBr(g)

at the anode,

    H.sub.2 O→H.sup.+ +OH.sup.-

    4OH.sup.- →O.sub.2 (g)+2H.sub.2 O+4e.sup.-

Essential to the invention is the use of a pool of bromine surrounding ametal electrode such as platinum or titanium in the electrolytic cell,providing a steady supply of bromide ions to combine with the hydrogenions evolved in the anodic portion of the cell. Electron conductivityfrom the electrode through the bromine is made possible by the bromideions dissolved in the liquid bromine surrounding the electrode. It hasbeen discovered that by dissolving the bromide ions in the liquidbromine, the bromine becomes conductive allowing current to flow throughthe liquid bromine to an electrolyte solution contacting the liquidbromine. Absent the dissolved bromide compounds, there is no currentflow detectable through the liquid bromine and in fact the liquidbromine effectively insulates the electrode in that portion of the cell.The dissolved bromide ions in the liquid bromine serve to maintaincharge neutrality in the liquid bromine as electrons flow from theelectrode and combine with the liquid bromine. By such processadditional bromide ions are formed and flow out of the liquid bromineand into the electrolyte solution contacting the liquid bromine. Thesolvent for the electrolyte solution is water and the bromide ionsevolving from the liquid bromine dissolve in the water forming asaturated solution of bromide, which saturation is maintained throughoutthe period of operation of the cell. Without the bromide ions present inthe liquid bromine, a potential drop would not take place at theinterface of the electrode and the liquid bromine to convert bromine tothe bromide ion source necessary for effective use of the cell. Thecurrent flow takes place from the electrode on the water side of thecell to the bromine liquid containing the electrode. While any metalelectrode non-reactive with the bromine can be used, platinum andtitanium electrodes are preferred, with platinum electrodes the mostpreferred.

The amount of bromide ions necessary to conduct charge through thebromine liquid is easily determinable by one skilled in this art. Thebromide compound is simply added to the bromine until current flow isdetected at the anticipated cell potentials to be employed. The rate ofcurrent flow will depend on the amount of dissolved bromide, but caremust be exercised to prevent overvoltage problems which will adverselyaffect the efficiency of the system.

The hydrogen bromide and oxygen gases emitted by such process andapparatus complete a water splitting cycle which begins with productionof hydrogen and bromine by electrolysis of hydrogen bromide solutions,e.g., as in copending application Ser. No. 956,760. The hydrogen bromideproduced in the reaction of the instant application can be recycled toproduce hydrogen and bromine further providing the option oftransporting the hydrogen from the production site for use inoxygen-hydrogen fuel cells or other energy producers using hydrogenfuel. As can clearly be seen, by combining the processes of the instantapplication and the aforementioned copending application, a closed cyclesystem can be established by recycling the bromine produced in theaforementioned copending application and adding water as a source ofhydrogen ions to constantly recover chemical energy using radiantenergy. The cyclic nature of such a process can further be demonstratedby the following equations:

    2HBr→H.sub.2 +Br.sub.2 (aforementioned copending application)

    2H.sub.2 O+2Br.sub.2 →4HBr+O.sub.2 (instant application)

    2H.sub.2 O→2H.sub.2 +O.sub.2 (overall reaction)

Also, while the cells may be powered by an external power source such asa battery connecting the two metal electrodes, the external power sourcemay be eliminated and the metal anode replaced by a suitablesemiconducting electrode utilizing radiant energy, e.g., solar, to powerthe cell. Or a combination of radiant energy and external power sourcemay be employed to power the cell. In any arrangement, the powersupplied by either the semiconductor, external power source, orsemiconductor and external power source must provide sufficient energyto evolve the O₂ and HBr gases. This energy will depend on the standardcell potential of the particular electrolyte being employed and itsconcentration. Also, if radiant energy is chosen to at least partiallypower the system, the particular radiant energy source and thesemiconductor material must be matched, i.e., the wavelength of theradiant energy source must be shorter than the band gap radiationcharacteristic of the semiconductor used. For example, for a siliconsemiconductor any light source with wavelengths less than 11,000 A couldbe used to provide power to the system. And while solar energy is thepreferred source of radiation when a semiconducting electrode is beingused, other radiant energy sources can be used such as laser radiationor light emitting solid state diodes. For example, for an 18% solutionof H₂ SO₄ electrolyte, 0.17 volt would be the minimum voltage requiredto run the cell (note the Example). Therefore, either the battery orother external power source, the semiconductor-radiant energy source, orthe combination of the two would have to meet this threshold voltage inorder to operate the system effectively. And while voltages higher thanthis threshold voltage may be used, e.g., to increase the rate of HBrand O₂ production, the efficiency of the system must be considered basedon the additional voltage used.

As demonstrated by the accompanying figures, the cells of the presentinvention contain two chambers: one containing the bromine electrolyteand water and the other electrolyte and water, separated by a hydrogenion permeable membrane. On the anode side of the cell, the hydroxyl ionspresent by virtue of the water in the cell give up electrons ultimatelyforming oxygen gas and hydrogen ions. The oxygen is evolved as a gas onthe anode side of the cell while the hydrogen ions transport through thehydrogen ion permeable membrane, thus completing the circuit in thecell. Examples of hydrogen ion permeable membranes which can be used areNafion® (E. I. Dupont de Nemours and Co.), polyvinyl chloride,polytetrafluoroethylene and thin quartz. When the hydrogen ions combinewith the bromide ions present in the cathodic portion of the cell thehydrogen bromide gas is evolved and will be dissolved in the water untilthe water is saturated, at which time it will evolve as a gas and can becollected. As mentioned above, the hydrogen bromide thus produced can beutilized in the process and apparatus of copending application Ser. No.956,760 to provide hydrogen for running a fuel cell. Thus, thecombination of the two inventions are capable of providing a closedcycle system for running a fuel cell.

The source of the bromide ions can be any well known, dissolvablebromide salt, such as KBr, LiBr, NH₄ Br, CsBr, SrBr₂, or NaBr, withtetra-alkyl ammonium bromides such as tetrapropyl ammonium bromide beingpreferred. Salt mixtures can also be used.

The electrolyte in the anode chamber of the electrolytic cell can be anyelectrolyte which will not interfere with the O₂ or HBr generation,e.g., sulfuric or phosphoric acids, in a preferred concentration of 10molar. The electrolyte in the cathode chamber can be the same as in theanode chamber or a bromide electrolyte which will not interfere with theHBr generation, e.g., aforementioned bromide salts present in the liquidbromine also in a preferred concentration of 1 molar. While water is thepreferred solvent for the cell, the system is readily adaptable to othersolvents. For example, alcohols or amines may be used as solvents forthe system. If alcohol or amine solvents are employed it is preferred toadd at least small amounts of water to the system.

The Nernst equation which governs the cell potential relationshiprequired for electrolysis in this process can be described as follows:

    E=E°+0.059 log P.sub.O.sbsb.2 +0.059 log C.sub.Br- +0.059 log C.sub.H+

wherein

E°=standard cell potential for cell components (e.g., for this reaction0.17 volt),

P_(O).sbsb.2 =partial pressure of oxygen produced in the cell,

C_(H+) =molar concentration of hydrogen ion in the cell,

E=the threshold voltage or cell potential at which current begins toflow in the cell and significant amounts of O₂ and HBr begin to evolve.

The preferred parameters for efficient operation of the cell of thepresent invention are:

P_(O).sbsb.2 >0.05 psi

C_(Br-) >0.1%

C_(H+) <48%

A cell with such parameters can be efficiently run at temperaturesbetween about 0° and 100° C. Percents as recited throughout thedisclosure are percents by weight.

FIG. 1 discloses a preferred embodiment of the method and apparatus. Inthis figure, electrolytic cell 1 contains a water solution of a sulfuricacid electrolyte 2, and a Nafion membrane 3 into which has been placed aconsumable bromine electrode comprising a pool of liquid bromine 4surrounding a platinum electrode 5. Platinum electrode 6 and batterysource 7 complete the electric circuit. In operation, the electron flowis from platinum electrode 6 through battery source 7 to platinumelectrode 5 transporting electrons from the dissolved hydroxyl ions 8 tothe bromine liquid 4. This causes the evolution of oxygen gas 9 andbromide ions 10, and the flow of hydrogen ions 11 through Nafionmembrane 3 to unite with the bromide ions 10 results in the evolution ofhydrogen bromide gas 12 which dissolves in the solution up to saturationpoint before evolving as a gas.

In FIG. 2, an alternative apparatus and method are shown where on thecathode side of the cell the platinum electrode 13 surrounded by bromineliquid 14 and sulfuric acid electrolyte solution 15 are separated fromthe anode side of the cell by Nafion membrane 16. But on the anode sideof the cell a semiconductor electrode such as gallium arsenide 17 isutilized as the power source connected by conductor 18 to the platinumelectrode 13. In the operation of such cell, a source of visible lightsuch as sunlight 19 impinges on the semiconductor electrode 17 causingcurrent flow to platinum electrode 13. The remainder of the method cycleis the same, with the hydrogen bromide 20 being evolved on the cathodeside of the cell and the oxygen gas 21 being evolved on the anode sideof the cell with appropriate ion transport through Nafion membrane 16.The semiconductor may be any suitable semiconductor depending on thewavelength of light impinging. For example, a silicon semiconductorwould be useful with light wavelengths up to 11,000 A while the TiO₂semiconductor would be useful with wavelengths up to about 4,000 A, anda GaAs semiconductor up to 8,900 A.

A third version of apparatus useful for performing the method of theinvention uses a combination of the two apparatuses of FIGS. 1 and 2,i.e., a combination of semiconductor and battery to power the cell.

EXAMPLE

In the apparatus of FIG. 1 utilizing two platinum electrodes and aNafion membrane, an 18% by weight solution of H₂ SO₄ in water was usedas the electrolyte in both compartments. A 4 molar solution oftetrapropyl ammonium bromide in liquid bromine was added to the cathodecompartment completely covering the platinum electrode (about 14 gmequaling about 5 cc). Upon application of a voltage of 0.17 volt acrossthe platinum electrode, oxygen gas began bubbling at the anode and HBrwas produced at the cathode.

Although this invention has been shown and described with respect to apreferred embodiment thereof, it should be understood by those skilledin the art that various changes and omissions in the form and detailthereof may be made therein without departing from the spirit and scopeof the invention.

Having thus described a typical embodiment of my invention, that which Iclaim as new and desire to secure by Letters Patent of the United Statesis:
 1. In the production of hydrogen bromide in an electrolytic cellhaving anode and cathode containing compartments separated by hydrogenion permeable membrane and containing an aqueous electrolyte solution,by causing current to flow from the anode to the cathode with theaccompanying ion transport through the hydrogen ion permeable membrane,the improvement comprising:utilizing as the cathode a metal electrodesurrounded by liquid bromine having dissolved therein bromide ions in anamount sufficient to conduct charge through the liquid bromine.
 2. Theprocess of claim 1 wherein both anode and cathode are metal electrodes.3. The process of claim 2 wherein at least one metal electrode isplatinum.
 4. The process of claim 2 wherein at least one metal electrodeis titanium.
 5. The process of claim 1 wherein the anode is asemiconducting electrode and at least part of the power to run the cellis provided by radiant energy.
 6. The process of claim 5 wherein thesemiconducting electrode is selected from the group consisting ofgallium arsenide, silicon and titanium dioxide.
 7. The process of claim1 wherein the bromide ions are provided by dissolving in the bromine acompound selected from the group consisting of tetrapropyl ammoniumbromide, potassium bromide, sodium bromide, ammonium brmide, cesiumbromide, strontium bromide, lithium bromide, and mixtures thereof. 8.The process of claim 1 wherein the anode and cathode containingcompartments contain sulfuric or phosphoric acid electrolyte in solutionin water.
 9. The process of claim 1 wherein the anode containingcompartment contains sulfuric or phosphoric acid electrolyte in solutionin water and the cathode containing compartment contains a bromidecompound electrolyte in solution in water.
 10. The process of claim 9wherein the bromide compound is selected from the group consisting oftetrapropyl ammonium bromide, potassium bromide, sodium bromide,ammonium bromide, cesium bromide, strontium bromide, lithium bromide,and mixtures thereof.